Just add to your child theme's functions.php, no need to mess around w/ the core thematic files... that is the beauty of the whole setup.
This will delete the thematic access from its current hook.
function remove_access() {
remove_action('thematic_header','thematic_access',9);
}
add_action('init','remove_access');
I always check this diagram when I need to figure out where stuff goes:
http://bluemandala.com/thematic/thematic-structure.html
To put it above the header div then you could add it to thematic_aboveheader() hook.
add_action('thematic_aboveheader','thematic_access');
Or to put it inside the header but before the branding, I think you'd need to move thematic_brandingopen() from hook 1 to hook 2 (see that diagram again).
This will delete the thematic access and brandingopen from their current hooks.
function remove_defaults() {
remove_action('thematic_header','thematic_brandingopen,1);
remove_action('thematic_header','thematic_access',9);
}
add_action('init','remove_defaults');
add_action('thematic_header','thematic_access',1);
add_action('thematic_header','thematic_brandingopen',2);